Stem Cell Research

Last update: October 2011
Contact:  Theresia Volhard

Authors

This issue of 'In Focus' was commissioned by the Stem Cell Network North Rhine Westphalia (see http://www.stemcells.nrw.de/en.html).

I. Medical and Scientific Aspects

What are stem cells?

The term 'stem cells' covers a non-uniform group of cells which, at least, share the following two properties:

• Stem cells are precursor cells of highly differentiated cells.
• After the stem cells have divided, the daughter cells can be either stem cells again (capable of self-renewal) or can differentiate into specific tissue, e.g. cardiac, neuronal, skin or muscle cells.

Stem cells can be identified first in the process of early embryonic development. The zygote is a totipotent (see module Totipotency and Pluripotency) stem cell (fig. 1) (see module Early Embryonic Development). It develops in the early embryonic stages (see module Early Embryonic Development) and forms the basis for all human tissue that is developed at later stages through specialisation or "differentiation". The further the specialisation process of the daughter cells of a stem cell advances, the more the spectrum of their differentiation potential into various types of tissue is restricted.

Stem cells also go on to exist throughout the human life span in various types of adult human tissue, playing an important role in tissue regeneration and repair. They maintain the functionality of tissues and organs by supplying differentiated cells to replace damaged or dead cells.

The classification and identification of stem cells is not entirely consistent and, therefore, may easily lead to misunderstandings. Stem cells are classified and identified either according to their potentiality (see module Totipotency and Pluripotency) or, as is more common, according to their derivation (see module Derivation of human embryonic stem cells). In everyday language, a distinction has emerged between adult and embryonic stem cells. However, from a scientific point of view it would be more accurate here to talk of tissue-specific cells instead of adult cells as opposed to stem cells which, depending on their origin, are described as

• EC cells (embryonic carcinoma cells) from embryonic tumour cells,
• EG cells (embryonic germ cells) from foetal precursor cells of gametes,
• ES cells (embryonic stem cells) from early embryonic stages (blastocysts).
  

The derivation of embryonic stem cells from blastocysts, during which the early embryo is destroyed, is ethically highly controversial.

The following properties of embryonic stem cells have been identified:

• In cell culture embryonic stem cells are capable of infinite division.
• Their chromosome number remains stable.
• Under suitable conditions, they are capable of developing into all types of body tissue - i.e. they are pluripotent (see module Totipotency and Pluripotency).
• How are human embryonic stem cells derived from blastocysts?

Currently, the technique most often used for the derivation of embryonic stem cells is that of in vitro fertilisation (IVF) (see module In vitro Fertilisation (IVF)). The application of this technique has become an established procedure in reproductive medicine as a way of inducing pregnancy in cases of unwanted childlessness. During the infertility treatment, test tube embryos are directly inserted into the woman`s uterus by catheter where they can then develop into a child (fig. 2) (see module Embryonic transfer after IVF). Early embryos created in vitro can, however, also be used for the derivation of embryonic stem cell lines.

Five to six days after fertilisation (see module Early Embryonic Development) the zygote has matured into a blastocyst. It consists of an outer cell layer - the trophoblast, which forms the basis the foetal part of the placenta - and of the inner cell mass which develops into the foetus.

In order to derive stem cells (fig. 3) (see module Derivation of human embryonic stem cells), the trophoblast is destroyed either by using antibodies or laser technology. This renders any further development of the embryo impossible. The inner cell mass, which is now accessible, is placed and cultivated in a special culture medium in a culture dish. These cell culturing conditions allow for continued growth of cells without their further differentiation. This is the basis from which embryonic stem cells can be developed.

Up to now, there is no method capable of allowing both the derivation of embryonic stem cells and, at the same time, maintaining the integrity and developmental capacity of the embryo.

Furthermore, abroad there is intense research on educing human embryonic stem cells from previously cloned embryos (see http://www.drze.de/in-focus/research-cloning). This technique did not yet succeed.

There are various imaginable ways of in vitro creation of blastocysts to be used for the derivation of embryonic stem cells. Accordingly, embryonic stem cells are subdivided into the following groups:

• ES cells generated from blastocysts created by in vitro fertilisation (IVF) (see module In vitro Fertilisation (IVF)) (fig. 4) (see module Development into a blastocyst after IVF).
• ES cells generated from blastcysts created by somatic cell nuclear transfer (SCNT) (so-called research or therapeutic cloning) (fig. 5) (see module Somatic Cell Nuclear Transfer (SCNT)).

Of these two methods only the IVF has already been successfully applied in humans in order to establish embryonic stem cell lines. These cell lines mainly differ from SNCT-Stem-cells in respect of their immune tolerance. In case of transplantations involving tissue derived from IVF embryonic stem cells, severe rejection reactions are very likely, similar to those triggered by foreign tissue grafts. In case of transplantations involving tissue derived from SCNT embryonic stem cells, so-called therapeutic cloning (see http://www.drze.de/in-focus/research-cloning), on the other hand, no or only minimal rejection reactions are anticipated provided the cell nucleus donor and tissue recipient would be genetically identical.

The SCNT cell nuclear transfer method could, in principle, also be used for "reproductive cloning", as has been shown in experiments with some species of mammals, where embryos from cell nuclear transfer were implanted in the uterus. The first successful experiment of this type was the creation of Dolly, the cloned sheep. However, this method is linked to high malformation and mortality rates.

 
What are the goals of research involving human embryonic stem cells?

Human embryonic stem cells are of great interest both for basic research and for clinical research. Given their capacity for unlimited proliferation, it is assumed that they are an inexhaustible source for the derivation of cell and tissue substitutes. Because of their differentiation potential, they are also suitable objects of research in order to examine numerous development processes in detail.

In the context of basic research, the main focus lies on gaining insight into the molecular mechanisms of individual cell specialisation as well as on examining the organisation of cells in situ. Furthermore, the goal is an improved understanding of the development and regulation of early stem cell stages as well as of the mechanisms behind the ability to proliferate and differentiate.

In the context of clinical research there are hopes that embryonic stem cells can be used to help in the creation of tissue substitutes, in particular in the case of tissues which have only limited or no natural capacity for renewal, such as neuronal tissue. The current debate concentrates on the use of embryonic stem cells in the treatment of diseases such as Parkinson Disease and Type I Diabetes, as well as diseases of the cardiovascular system. It is also conceivable that embryonic stem cells may be genetically modified and could then be used in gene therapy, for example, to restore a destroyed immune system as in the case of the HIV Aids disease (see http://stm.sciencemag.org/content/2/36/36ra43.abstract?sid=dedaab31-25dd-4b78-a856-fd55b340fcf5).

 
What are the latest developments in research?

Since the derivation of the first human embryonic stem cell lines in 1998, research in this field has only made slow progress. All the same, initial results on the in vitro differentiation of human embryonic stem cells are available. Up to now, efforts to generate various cell types from human embryonic stem cells have been successfully accomplished in the case of neuronal cells (see module Neuronal Cells), cardiac muscle cells, vascular cells (see module Cardiac and Vascular Cells), blood cells (see module Blood Cells), pancreatic cells, hepatic cells and trophoblast cells (see module Hepatic, Pancreatic and Trophoblast Cells). The allocation of the precursor cells to one of these groups of tissue did not generally take place on the basis of the determination of their functionality, but by determining the (surface) markers formed by the cells. In a small number of cases precursor cells derived from human embryonic stem cells were transplanted into model organisms, such as mice or chicken. There is no incidence, however, of a functional involvement of those cells in complex tissue structures. In addition, the derivation of sperm from murine embryonic stem cells (see module Sperm From Murine Stem Cells) has recently been described.

Furthermore, in 2008, two groups of researchers recently published independently of each other techniques to reprogram human somatic cells (see module  Reprogramming) successfully so that they show essential characteristics of embryonic stem cells. Such cells are called induced pluripotent stem cells (iPS-cells). In October 2009, the Berlin-Brandeburg Academy of the Sciences (Berlin-Brandenburgische Akademie der Wissenschaften) together with the National Academy of the Sciences (Nationale Akademie der Wissenschaften (Leopoldina)) published a statement (see module Statement by the Berlin-Brandenburg Academy of the Sciences and the National Academy of the Sciences) concerning this new technique of stem cell derivation.

Obtaining embryonic stem cells after cell nuclear transfer (so-called "cloning for research purposes" or "therapeutic cloning") has proven to be technically very difficult in humans - leaving aside the ethical and legal problems - and attempts to date have not been successful. In 2004 the group of researchers led by the Korean veterinary surgeon Hwang (see module The Hwang Fraud) had reported the successful cloning of human cells. However, this proved to be fraudulent.

In innovative lines of research scientists try to derive human embryonic stem cells without making use of cloning techniques. By parthenogenesis (see module Parthenogenesis) a human egg cell shall be encouraged to divide without adding sperm. Out of embryos developing that way stem cells can be derived, which can differentiate in many different kinds of cells again. So far this procedure was successfully performed with mice.

Additionally in other lines of research scientists try to avoid possible ethic concerns regarding the usage of embryos when deriving stem cells. For this purpose stem cells shall be cultivated, which beforehand got isolated from amniotic fluid (see module Stem Cells from Amniotic Fluid). This is the description, that out of such cells human fat, muscle, nerve and liver cells were derived.

In October 2010 surgeons in Shepherd Center in Atlanta, USA, treated the first partially paralysed patient with embryonic stem cells. The trial, which is sponsored by Geron Corp., a biotec company located in California, has been part of a phase 1 trial which involves 10 patients in seven sites in the USA. The study is primarily aimed at testing whether the therapy is safe. In addition it shall be tested whether the treatment restores sensation and enables the patient to regain movement. In the treatment, which is known as GRNOPC1, the surgeons inject the patient with about 2 million oligodendrocyte progenitor cells, a type of cell that produces myelin, a coating that allows impulses to move along nerves.  To date it is unknown whether the trial will prove successful.

 
What are the open questions and problems?

For both embryonic stem cells and adult stem cells, the following criteria must be considered with regard to their possible regular clinical applications:

• Proliferability: it must be possible to proliferate the stem cells in sufficient numbers.
• Differentiability: it must be possible to stimulate their differentiation into the required cell types.
• Purity: it must be possible to generate differentiated cells of one cell type only; i.e. pure cells instead of cell mixtures.
• Targeted integratability: it must be possible to transplant the cell or tissue replacement into the correct part of the body.
• Ruling out tumour formation: it must be guaranteed that there is no uncontrolled transplant growth or risk of tumour formation.
• Long-term therapeutic efficacy: the transplants must prove their functionality in the organism and their therapeutic effects on a long-term basis.
• Immune tolerance: the cell transplants should not be rejected by the immune system of the transplant recipient.

Research on human embryonic stem cells is still at its very beginning. Finding answers to the following questions arising from basic research is the precondition for any application in regenerative medicine:

• How can embryonic stem cells be derived efficiently?
• Do all embryonic stem cell lines show the same properties?
• How can embryonic stem cells be genetically modified?
• How can the differentiation of daughter cells be regulated?
• What new methods and tools are needed in order to measure and control this differentiation in vivo and in vitro?

 
Are there alternatives to research involving human embryonic stem cells?

Tissue-specific adult stem cells, including stem cells from umbilical cord blood, are discussed as the main alternatives to research on human embryonic stem cells.

In the context of basic research the main focus lies on the cultivation, differentiation and manipulation of human embryonic stem cells. These specific properties can only be found in embryonic stem cell lines; adult stem cells are not viable alternatives in this area.

To date, a number of therapeutic methods have emerged from the field of tissue replacement from adult stem cells. Some of them have already been successfully applied to clinical uses for a long time, e.g. bone marrow transplantation in the wake of radiotherapy (haemopoietic stem cells) or skin regeneration after burns (dermatogenic stem cells). However, this cannot lead to the conclusion that adult stem are particularly well suited for the realisation of the clinical research goals mentioned above. For the assessment of their suitability the above-mentioned criteria of proliferability, purity, differentiability, etc. are the determining factors.

From today' s perspective it can be assumed that adult stem cells are, in terms of purity, risk of tumour formation and immune tolerance, better suited for clinical uses than embryonic stem cells. On the other hand, in terms of proliferability and differentiability, embryonic stem cells offer advantages. With regard to immune tolerance, embryonic stem cells from SCNT blastocysts are very likely to be similar to adult stem cells. Currently it is not possible to give a substantiated estimation from a scientific point of view which would give one of the two lines of research priority over the other in terms of their clinical application opportunities.

Next: II. Selected National and International Laws and Regulations